Conventional techniques of the type described above include those disclosed in Patent Literatures 1 to 5.
Patent Literature 1 discloses a technique relating to a phase switching device for a three-phase electric motor. The technique has three phase-detecting circuits detecting interphase signals at the zero-cross timing, and a control unit detecting an open phase by detecting whether or not there is a signal from the phase-detecting circuits. The control unit also judges the order in which the signals rise or fall to judge whether or not the three-phase alternating current is a positive-phase sequence current.
Patent Literature 2 discloses a technique relating to a circuit for detecting a reverse phase of a three-phase power supply. The technique has a signal generating circuit comprising a first photocoupler and a second photocoupler. The first photocoupler is activated by an R-S interphase voltage of a load input circuit connected to a three-phase power supply having an R phase, an S phase, and a T phase. The second photocoupler is activated by an R-T interphase voltage of the load input circuit. When an output signal from either one of the two photocouplers rises or falls, a judgment is made as to whether or not there is an output signal from the other photocoupler, thereby detecting an improper connection in the three-phase power supply.
Patent Literature 3 discloses a technique relating to an apparatus for detecting a frequency, open phase and reverse phase of an alternating-current power supply. The technique has a photocoupler activated by the alternating-current power supply, and a pulse duration detecting means detecting a pulse duration from the rise to the fall of an output signal from the photocoupler. When the pulse duration detected by the pulse duration detecting means is not within an allowable value, it is judged that the alternating-current power supply has noise imposed thereon.
Patent Literature 4 discloses a technique relating to an apparatus for detecting a reverse phase of a three-phase alternating-current power supply. The technique is provided with a voltage input means digitizing and inputting an alternating-current voltage signal of each phase after lowering the signal to a predetermined level, a phase signal generating means generating a power supply phase signal indicating a phase shift of each phase relative to a predetermined phase on the basis of the alternating-current voltage signal of each phase input by the voltage input means, a lead direction detecting means detecting a lead direction of the power supply phase signal generated by the phase signal generating means, and a reverse phase judging means judging a reverse-phase condition of the three-phase alternating-current power supply on the basis of the lead direction of the power supply phase signal detected by the lead direction detecting means.
Patent Literature 5 discloses a technique relating to an apparatus for detecting an abnormality in a driving circuit. The technique has an abnormality detecting circuit connected in parallel to a semiconductor device and formed to generate a pulse signal synchronized with an alternating-current power supply when the semiconductor device is in the state of cutting off a load current, a discriminating signal detecting circuit discriminating between an electric current flowing through the abnormality detecting circuit for the semiconductor device and the load current, and a judging circuit judging the content of abnormality from output signals from the abnormality detecting circuit and the discriminating signal detecting circuit.